1. Field of the Invention
The invention relates generally to extending the range of electrically powered vehicles, without compromising their power control characteristics. More particularly, this invention relates to an apparatus including a direct current (DC) electric motor coupled to a drive wheel via a continuously variable transmission. Specifically, the invention relates to the inclusion, of a belt type continuously variable transmission (CVT) coupled to a variable speed DC electric motor, as part of an electric storage powered vehicle, and a method for their use.
2. Description of the Prior Art
It has been recognized that there are desirable features to using electric motors to power land vehicles of many sizes and uses. That electric motors produce no gaseous emissions is among the desirable features. For electrically powered vehicles to be unconstrained, by constant connection to external electrical power sources and to have the freedom of movement of petroleum powered vehicles, electric vehicles must have self contained, or otherwise mobile, electric power storage. These are commonly in the form of on-board electric storage batteries. However, electric vehicles, having self contained electric power storage, still remained constrained by a useable range between recharges, of electrical power, that are commonly regarded as too limited.
The efforts to increase the range of such vehicles have included increasing the electrical energy storage capacity of the electrical storage devices or means, and improving the efficiency of the power drive of the vehicle. One effort, at improving power drive efficiency, has involved the use of alternating current (AC) motors. Designs for vehicles having such motors have included complex motor control systems that involve the conversion of the DC source of power, from the electric storage device, to AC, and varying the alternating frequency of the converted power to vary the speed of rotational fields within the AC motor. Not only are such variable rotational field control systems relatively complex, they also introduce additional energy loss into the power system of the vehicle, reducing overall efficiency.
U.S. Pat. No. 5,355,749 discloses the use of a variable rotational field control system. It further discloses the use of a belt type CVT, between the output shaft of the motor and a drive wheel of the vehicle. The variable rotational control system controls the input to the motor. While, the CVT controls the load imposed upon the motor. In this manner a target operational pattern for the motor is approached, toward the goal of improving the efficiency of the power drive system.
Another effort has used a DC motor coupled to a belt type CVT. U.S. Pat. No. 3,202,234 discloses the use of a shunt wound DC motor coupled to a drive wheel, via a CVT, where the CVT also performs a clutching function.
A common configuration for a belt type CVT, well known in the art, involves two pulleys, a driver pulley connected to the mechanical power source and a driven pulley connected to the load. Each pulley has opposing faces angled to the axis of rotation in a manner that supports the working surfaces of a power transmission V-belt with a trapezoidal cross section. The belt resides between the two faces at a radius from the axis of rotation defined by the point at which the width of the belt fills the gap between the two halves. The two faces are moveable toward or away from each other and thereby affect the size of the gap and thus the radius. The combination of the two radii and separation of the pulleys correspond to the length of the belt. Since the separation is fixed, as one radius is shortened the other must correspondingly be lengthened, and in such a manner as to maintain tension over the length of the belt. The ratio of each radius of the respective pulleys effects the speed ratio of the CVT. The speed ratio is the ratio of the output rotational speed to the input rotational speed.
In this reference it appears that the faces on the driver pulley are biased to be away from each other far enough that the width of the belt cannot bridge the gap. Accordingly, it acts as a clutch that allows the DC motor to spin freely such as at speed-no-load while the vehicle is a rest. When the operator desires to have the vehicle move, the operator depresses a throttle pedal which first causes the full electric storage device potential to be placed across the electrical input of the motor, causing it to accelerate toward speed-no-load. For as long as the throttle pedal is depressed any amount necessary to cause the vehicle to move, the full electrical potential is connected across the motor electrical input. Continued pedal depression causes the driver pulley faces to move toward each other, closing the gap, and squeezing the belt so as to establish tension upon the belt which leads to transmitting power to the belt.
In contrast, it appears that the faces of the driven pulley are biased toward each other for the most narrow gap available. Thus, at the point where tension is first placed upon the belt, the belt is finding a minimum radius at the driver pulley, and a maximum radius at the driven pulley, leading to a minimum speed ratio, or lowest gear ranges, for the CVT. Apparently, as additional pressure is placed upon the pedal, the gap for the driver pulley is further narrowed, causing the belt to be under additional tension, causing the bias in the driven pulley to be partially overcome, causing the gap for the driven pulley to widen, and ultimately leading to an increasing speed ratio or higher gear range. It appears that it is through this process, of going from a clutched condition, to a low speed ratio, and onto increasingly higher speed ratios, that the acceleration and speed of the vehicle is controlled. This process allows the shunt wound DC motor to accelerate the vehicle from a standing start without having to operate in the relatively inefficient condition of low rotational speed and high load. Further, it appears to allow the motor to generally avoid operation at low rotational speed with high load, when the vehicle is operated over flat and level surfaces.
However, the approach disclosed in "the '234" patent is fraught with difficulties and leaves other room for improvement from both efficiency of operation and flexibility of design stand points. The approach of the '234 patent does not appear to seek an optimum operational efficiency, but merely to avoid one operational condition that is particularly inefficient. Belt drives that are clutched in the disclosed manner tend to be unacceptably rough. There is apparently no way to maintain a constant vehicle speed, at speeds which are slower than would be produced by full motor rotational speed through the lowest speed ratio, without continuous slippage of the belt against the pulleys. This approach relies solely upon the use of a shunt wound motor. A series wound motor would be completely unsuitable, as it would go into an overspeed condition when allowed to experience full electric storage device potential with no load on the output shaft.
Further, it would appear that throttle operation is counterintuitive. Apparently, accelerating from a stop over a flat and level surface would be fairly ordinary, if the operator exercises a certain amount of restraint. The operator presses continuously farther on the pedal, to raise the speed ratio, which puts more load upon the motor, which causes the motor to draw more power from the electric storage device as its rotational speed is drawn down by the load. However, it also appears that, even in the flat and level situation, if the operator presses too suddenly upon the pedal, the speed ratio would rise more rapidly than the motor can accommodate and the rotational speed sinks into the operation region of low rotation speed and high load, that is sought to be avoided.
Of even greater concern, there appears to be at least one condition, commonly to be encountered, where throttle movement would actually be the reverse of what an operator would likely expect. If the vehicle approaches a positive incline at a throttle setting between minimum and maximum, an operator might normally expect to add additional pressure upon the throttle pedal to maintain adequate power to climb, at about constant speed. However, additional pressure for the approach of '234 causes the speed ratio to increase, motor speed to decrease and torque applied to the drive wheel to decrease, exacerbating the reduction of speed brought on by the presence of the incline. If taken to its limit one can reasonably expect the motor to be drawn down to a stalled or locked rotor condition. Only by reducing throttle pressure can the speed ratio be lowered to allow the motor to continue at the desired operating speed, and allow the vehicle to continue up the incline without stalling.
Accordingly, there remains the need to produce an electric vehicle drive that demonstrates improved efficiency, utilizing a DC motor to avoid motor control schemes that arc relatively complex and introduce energy losses to the system, while not compromising the characteristics of smooth power application, flexibility of design, and intuitive throttle operation.